The pharmacokinetics of a single oral or rectal dose of concurrently administered isoniazid, rifampin, pyrazinamide, and ethambutol in Asian elephants (Elephas maximus)

Tuberculosis, caused by Mycobacterium tuberculosis, is a disease of concern in captive Asian elephants (Elephas maximus). Treatment for tuberculosis in elephants utilizes multidrug protocols combining isoniazid, rifampin, pyrazinamide, and/or ethambutol. In this study, a single, coformulated dose of isoniazid 5 mg/kg, rifampin 10 mg/kg, pyrazinamide 30 mg/kg, and ethambutol 30 mg/kg was administered orally to six Asian elephants, and rectally to five elephants using a cross-over design. Blood samples were collected serially over 24 h. Pyrazinamide and ethambutol concentrations were determined using validated gas chromatography assays. Isoniazid and rifampin concentrations were determined using validated high-performance liquid chromatography assays. Rectal isoniazid produced an earlier Tmax compared with oral administration. Oral isoniazid resulted in a comparatively lower Cmax , but higher AUC values compared with rectal isoniazid. Oral rifampin and oral ethambutol were well absorbed while rectal rifampin was not. Oral pyrazinamide produced comparatively higher Cmax and AUC values compared with rectal pyrazinamide. Results of this study indicate that currently recommended therapeutic monitoring sample collection times for rectal isoniazid and oral rifampin do not provide an accurate assessment of exposure for these drugs. This study demonstrates notable individual variability, indicating that dosing of these medications requires individual monitoring and provides additional information to guide the clinician when treating elephants.

Workshop report: the 2012 antimicrobial agents in veterinary medicine: exploring the consequences of antimicrobial drug use: a 3-D approach

Antimicrobial resistance is a global challenge that impacts both human and veterinary health care. The resilience of microbes is reflected in their ability to adapt and survive in spite of our best efforts to constrain their infectious capabilities. As science advances, many of the mechanisms for microbial survival and resistance element transfer have been identified. During the 2012 meeting of Antimicrobial Agents in Veterinary Medicine (AAVM), experts provided insights on such issues as use vs. resistance, the available tools for supporting appropriate drug use, the importance of meeting the therapeutic needs within the domestic animal health care, and the requirements associated with food safety and food security. This report aims to provide a summary of the presentations and discussions occurring during the 2012 AAVM with the goal of stimulating future discussions and enhancing the opportunity to establish creative and sustainable solutions that will guarantee the availability of an effective therapeutic arsenal for veterinary species.

Establishing bioequivalence of veterinary premixes (Type A medicated articles)

a) Key issues concerning Premix (Type A medicated articles) Bioequivalence evaluations: 1) This is a complex issue concerning both route of administration and formulation. 2) If the animal is not at the bunk/trough, the animal is not self-administering (eating medicated feed), thus there can be no drug absorption. b) Differing opinions among scientists and regulatory authorities/expert bodies regarding: 1) No harmonization on how to design, conduct, and interpret in vivo studies. 2) Applicability of biowaivers to Type A (premix) products. 3) Why are topdress and complete feed considered differently? Are they different formulations or different routes of administration? 4) Single dose vs. multi-dose studies. 5) What is the final formulation? c) What are the next steps: 1) Harmonize current bioequivalence guidelines through the VICH process. 2) Determine the applicability/non-applicability of the Biopharmaceutical Classification System (BCS). 3) Establish the Total Mixed Ration (i.e. formulation) effects. 4) Define the test subject (individual, pen, etc.).

European field study of the efficacy and safety of the novel anthelmintic monepantel in sheep

During 2007, a large-scale controlled, multicentre, blinded and randomised field study was conducted in Scotland, England and France to assess the efficacy and safety of monepantel, the first molecule to be developed from the recently discovered amino-acetonitrile derivatives class of anthelmintics, in sheep. Monepantel was administered orally, at a minimum dose of 2.5 mg/kg bodyweight, for the control of gastrointestinal nematodes in sheep maintained at pasture in a range of commercial production systems. Efficacy was measured by faecal egg count (FEC) reduction tests seven days after treatment and was demonstrated to be over 98 per cent against mixed-genus infections. The reduction in FEC of monepantel-treated sheep was statistically significantly greater than in untreated control sheep (P<0.0001). The efficacy of monepantel against mixed-genus natural field infections of the major gastrointestinal nematodes was in agreement with similar studies conducted in Australia and New Zealand. There were no treatment-related adverse events during the study, which included the use of a range of concomitant treatments.

Identification and comparison of marbofloxacin metabolites from the plasma of ball pythons (Python regius) and blue and gold macaws (Ara ararauna)

Marbofloxacin is a veterinary only, synthetic, broad spectrum fluoroquinolone antimicrobial agent. In mammals, approximately 40% of the oral dose of marbofloxacin is excreted unchanged in the urine; the remaining is excreted via the bile as unchanged drug in the feces. The Vd ranges from 1.1 (cattle) to 1.3 (dog, goat, swine) L/kg. Because of extra-label use of marbofloxacin in birds and reptiles, this study was designed to determine the profile of metabolites in plasma and compare the circulating metabolite profile between a reptile and an avian species. Six adult ball pythons (Python regius) and 10 blue and gold macaws (Ara ararauna) were used in this study. The macaws were dosed both i.v. and p.o. with a single 2.5 mg/kg administration where as the pythons received a single 10 mg/kg dose both i.v. and p.o. The metabolite profiles of marbofloxacin in the plasma of these species were determined using a high performance liquid chromatography system with a mass spectrometer for detection (LC/MS/MS). Mass spectra data generated from the snake and bird plasma samples were compared with previously reported LC/MS/MS mass spectral data. Evidence does not suggest differences due to route of administration (i.v. vs. p.o.) in either species. Four chromatographic peaks with resulting daughter spectrum were identified and represent 12 possible metabolite structures. All of the proposed metabolites, except for the N-oxide, appear to be unique to macaws. The potential metabolites identified in macaws appear to be very different than those reported for chickens.

Interspecies allometric scaling: prediction of clearance in large animal species: Part II: mathematical considerations

Interspecies scaling is a useful tool for the prediction of pharmacokinetic parameters from animals to humans, and it is often used for estimating a first-time in human dose. However, it is important to appreciate the mathematical underpinnings of this scaling procedure when using it to predict pharmacokinetic parameter values across animal species. When cautiously applied, allometry can be a tool for estimating clearance in veterinary species for the purpose of dosage selection. It is particularly valuable during the selection of dosages in large zoo animal species, such as elephants, large cats and camels, for which pharmacokinetic data are scant. In Part I, allometric predictions of clearance in large animal species were found to pose substantially greater risks of inaccuracies when compared with that observed for humans. In this report, we examine the factors influencing the accuracy of our clearance estimates from the perspective of the relationship between prediction error and such variables as the distribution of body weight values used in the regression analysis, the influence of a particular observation on the clearance estimate, and the 'goodness of fit' (R(2)) of the regression line. Ultimately, these considerations are used to generate recommendations regarding the data to be included in the allometric prediction of clearance in large animal species.

Interspecies allometric scaling. Part I: prediction of clearance in large animals

Interspecies scaling is a useful tool for the prediction of pharmacokinetic parameters from animals to humans, and it is often used for estimating a first-time in human dose. The knowledge of pharmacokinetics in veterinary species is important for dosage selection, particularly in the treatment of large zoo animal species, such as elephants, giant cats and camels, for which pharmacokinetic data are scant. Therefore, the accuracy in clearance predictions in large animal species, with and without the use of correction factors (rule of exponents), and the impact of species selection in the prediction of clearance in large animal species was examined. Based upon this analysis, it was determined that there is a much larger risk of inaccuracies in the clearance estimates in large animal species when compared with that observed for humans. Unlike in humans, for large animal species, correction factors could not be applied because there was no trend between the exponents of simple allometry and the appropriate correction factor for improving our predictions. Nevertheless, we did see an indication that the exponents of simple allometry may alert us as to when the predicted clearance in the large animal may be underestimated or overpredicted. For example, if a large animal is included in the scaling, the predicted clearance in a large animal should be considered overestimated if the exponent of simple allometry is >1.3. Despite the potential for extrapolation error, the reality is that allometric scaling is needed across many veterinary practice situations, and therefore will be used. For this reason, it is important to consider mechanisms for reducing the risk of extrapolation errors that can seriously affect target animal safety, therapeutic response, or the accuracy of withdrawal time predictions.

Pharmacokinetics and i.m. bioavailability of ceftiofur in Asian elephants (Elephas maximus)

Captive elephants are prone to infections of the feet, lungs, and skin. Often treatment regimens are established with no pharmacokinetic data on the agents being used for treatment in these species. A pharmacokinetic study using ceftiofur (1.1 mg/kg) was conducted in four adult female captive Asian elephants (Elephas maximus) at Busch Gardens in Tampa, Florida. Elephants were given both i.v. and i.m. administrations in a complete crossover design with a 3-week washout period between treatments. Blood samples were collected prior to drug administration and at 0.33, 0.67, 1, 1.5, 2, 4, 8, 12, 24, 48 and 72 h postadministration. Ceftiofur analysis was performed using a validated liquid chromatography/mass spectrophotometric (LC/MS) assay. Plasma concentrations for the i.m. samples were lower than expected. The mean C(max) following i.m. administration was 1.63 microg/mL with a corresponding T(max) of 0.55 h. Following i.v. administration, the median V(d(ss)) was 0.51 L/kg and a median Cl(p) of 0.069 L/kg/h. Mean i.m. bioavailability was 19%. The results indicate that ceftiofur used at 1.1 mg/kg i.m. could be useful in elephants when given two to three times a day or alternatively, 1.1 mg/kg i.v. once daily, depending upon the MIC of the pathogen.

Multiple dose pharmacokinetics and acute safety of piroxicam and cimetidine in the cat

The purpose of this study was to evaluate the multiple dose pharmacokinetics and acute safety of piroxicam and cimetidine alone and in combination in cats. Seven healthy cats were included in this randomized-crossover study. The cats were assigned to groups designated to receive cimetidine alone (15 mg/kg, p.o., q12 h), piroxicam alone (0.3 mg/kg, p.o., q24 h), and piroxicam combined with cimetidine (both at aforementioned doses). The cats were dosed for 10 days followed by at least a 2-week washout period between trials. Serial blood samples were collected following the first and last doses and analyzed utilizing a high-performance liquid chromatography with mass spectrometry detection (LC/MS) assay. Pharmacokinetic parameters were determined using noncompartmental analysis. Endoscopic evaluation of the gastric mucosa was performed and serum urea nitrogen (SUN), creatinine, alkaline phosphatase (ALP), and alanine transaminase (ALT) activities were evaluated. There were not a clinically relevant difference between the pharmacokinetic parameters of piroxicam administered alone or in combination with cimetidine after either the first or last dose. Gastric ulcers were not observed in any cats although gastric erosions were. The SUN, creatinine, ALP, and ALT activities remained within reference ranges for all cats. It appears that once daily, short-term use of piroxicam alone and in combination with cimetidine in cats is relatively safe based on the parameters evaluated in this study. However, further studies are necessary to determine the long-term gastrointestinal safety of piroxicam.

Simultaneous extraction and quantitation of fentanyl and norfentanyl from primate plasma with LC/MS detection

The quantitation of both fentanyl and its desalkyl metabolite, norfentanyl, in plasma using LC/MS has not been previously described. The detection and quantitation of fentanyl and norfentanyl was achieved using LC/MS detection. The liquid-liquid extraction used toluene as the organic phase. Chromatography was carried out using a Zirchrom-PBD (50 mm x 2.1 mm, 3 microm) column with a mobile phase of acetonitrile-ammonium acetate (10 mM), citrate (0.1 mM, pH 4.4) (45:55, v/v) with a flow rate of 0.3 ml/min. Mass spectroscopy detection was performed using ESI in the positive mode. The LOQ for fentanyl was 25 pg/ml and norfentanyl was 50 pg/ml. For the concentrations of 75, 250, and 750 pg/ml, respectively, fentanyl had inter-day precisions of 6.6, 7.2, and 6.6% with accuracies of 4.0, 5.1, and 5.1% and intra-day precisions of 1.6, 1.9, and 1.9% with accuracies of 11.6, 9.4, and 8.4%, and norfentanyl had inter-day precisions of 7.4, 0.3, and 0.7% with accuracies of 9.1, 8.8, and 12.3% and intra-day precisions of 5.3, 1.4, and 0.1% with accuracies of 10.9, 8.9, and 12.8%. The recoveries of fentanyl were 85, 92, and 75% and of norfentanyl were 40, 49, and 46% at the 75, 250, and 750 pg/ml concentrations, respectively.

Single dose pharmacokinetics of piroxicam in cats

Piroxicam (PIRO) is a nonsteroidal anti-inflammatory drug (NSAID) recognized for its value as a chemopreventative and anti-tumor agent. Eight cats were included in this study. PIRO was administered in a single oral (p.o.) and intravenous (i.v.) dose of 0.3 mg/kg. The study was designed as a randomized complete crossover with a 2-week washout period. Serial blood samples were collected after each dose and plasma was analyzed for PIRO. Pharmacokinetic parameters of PIRO were determined using noncompartmental analysis. PIRO is well absorbed in the cat with a median bioavailability (F) of 80% (range 64-124%). The median i.v. t1/2 was 12 h (range 8.6-14 h). The median Cmax was 519 ng/mL with a corresponding Tmax of 3 h. PIRO appears to be rapidly absorbed following p.o. administration in cats with a higher Cmax and AUC than in dogs.

Azithromycin metabolite identification in plasma, bile, and tissues of the ball python (Python regius)

Azithromycin is the first of a class of antibiotics classified as azalides. Six ball pythons (Python regius) were given a single dose of azithromycin at 10 mg/kg p.o. and i.v. in a crossover design. Serial blood samples were collected for unchanged azithromycin and to determine, if possible, the structure and number of circulating azithromycin metabolites. After a 4-month wash-out period, the snakes were given azithromycin p.o. as a single dose of 10 mg/kg for the study of azithromycin metabolism and metabolite tissue distribution. Bile, liver, lung, kidney, and skin samples were analyzed for the metabolites identified from the first experiment. Unchanged azithromycin accounted for 80, 68, and 60% of the total material at 12, 24, and 48 h postadministration in plasma, independent of route of administration. At both 24 and 72 h postadministration, azithromycin accounted for 70% of total azithromycin- associated material in bile. In liver and kidney, unchanged azithromycin accounted for 40% of the total azithromycin-associated material; this doubled in lung and skin. Fifteen metabolites were positively or tentatively identified in plasma, bile, or tissues of all snakes. Four of these possible metabolites: 3'-desamine-3-ene-azithromycin, descladinose dehydroxy-2-ene-azithromycin, 3'-desamine-3-ene descladinose-azithromycin, and 3'-N-nitroso,9a-N-desmethyl-azithromycin are unique to this species. Descladinose-azithromycin, 3'-N-desmethyl,9a-N-desmethyl-azithromycin, and 3'-N-desmethyl, 3'-O-desmethyl-azithromycin were the only metabolites identified in skin. Kidney tissue contained a greater number of metabolites than liver tissue, with 3'-N-didesmethyl-azithromycin being identified only in the kidney. Compared with the dog and cat, a greater number of metabolites were identified in ball python plasma. The percentage of unchanged azithromycin in bile is not different between the three species.

Effect of cimetidine on pharmacokinetics of orally administered cyclosporine in healthy dogs

OBJECTIVE

To describe the pharmacokinetics of cyclosporine (CyA) in healthy dogs after oral administration alone or in combination with orally administered cimetidine.

ANIMALS

10 healthy adult Beagles.

PROCEDURE

Dogs were randomly assigned to receive CyA alone or CyA in combination with cimetidine. After a washout period of 2 weeks, dogs then received the alternate treatment. The CyA plus cimetidine treatment required administration of cimetidine (15 mg/kg of body weight, PO, q 8 h) for 8 days and administration of CyA (5 mg/kg, PO, q 24 h) on days 6 through 8. The CyA treatment alone required administration of CyA (5 mg/kg, PO, q 24 h) for 3 days. On the third day of CyA administration during each treatment, blood samples were collected immediately before (time 0) and 0.5, 1, 1.5, 2, 2.5, 3, 5, 7, 9, 11, 13, 15, 21, and 24 hours after initiating CyA administration.

RESULTS

Time until maximum CyA concentration was significantly longer for CyA in combination with cimetidine. Assessment of estimated pharmacokinetic variables revealed a significantly faster rate of change in the distribution phase for CyA in combination with cimetidine. Maximum CyA concentration differed significantly among dogs but did not differ significantly between treatments.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of our data suggests that cimetidine may affect absorption of orally administered CyA, but overall, it does not affect the pharmacokinetics of CyA. There is considerable variability in the maximum concentration of CyA among dogs, and monitoring of blood concentrations of CyA during treatment is advised.

Cimetidine inhibits nitric oxide associated nitrate production in a soft-tissue inflammation model in the horse

Cimetidine (CIM) is an H2-receptor antagonist that has been used in racehorses in an attempt to reduce the occurrence of stress-related gastric ulceration. It has also been shown to produce several useful effects other than its gastric acid suppression properties. Further, it is a well documented antagonist of cytochrome P-450 (CYP) mediated oxygenation reactions. Nitric oxide (NO), a recently discovered mediator or modifier of numerous physiological functions, is generated by several forms of nitric oxide synthase (NOS), one of which is inducible (iNOS). Inducible NOS, expressed in neutrophils and macrophages as part of the inflammatory response to noxious stimuli, contains both a CYP and a CYP reductase domain. Because of the similarity of structure of iNOS and CYP, it was decided to determine whether CIM could reduce NO production, using a carrageenan inflammation model in the horse. Two experiments were conducted. In Trial 1, six female Thoroughbred horses each had three tissue chambers inserted subcutaneously on the sides of the neck. The study was divided into three treatments: 0.9% NaCl (NaCI), CIM (3 mg/kg), and aminoguanidine (AG; 25 mg/kg), an inhibitor of iNOS. Each mare received three i.v. injections 12 h apart prior to instillation of 1 mL of carrageenan into the test chamber. Blood and tissue chamber fluid (TCF) were collected serially. Concentrations of NO3- (the major metabolite of NO), albumin, total protein, CIM and AG were measured and complete cell counts and differentials were conducted. Trial 2 also used six female Thoroughbred horses implanted with at least two tissue chambers inserted subcutaneously on the sides of the neck. The study was divided into two treatments: NaCl (0.9%) and CIM (6 mg/kg). Each mare received seven i.v. injections of either NaCl or CIM 8 h apart prior to instillation of 1 mL of carrageenan into the test chamber. Blood and TCF were collected serially as before, and analysed for NO3- and CIM content. Areas under the curve (AUC) of the different parameters were calculated for the periods of -1-1, -1-3 and -1-7 days (Trial 1) and -2-1 for Trial 2. Absolute values were also compared at 4, 8 and 12 h postcarrageenan. Saline treatment did not reduce the elevated concentrations of NO3- in either plasma or TCF. Plasma, test chamber and control chamber NO3-concentrations rose from 0 to 12 h, and were very similar in all three sampled fluids. Cimetidine significantly (P< or =0.05) decreased NO3- production in plasma over the periods of -1-1, -1-3, and -1-7 days post inflammation when compared to NaCl treatment in Trial 1. Aminoguanidine and CIM decreased NO3-production in TCF for the periods -1-1, 1-3, and -1-7 days post inflammation in Trial 1 and -2-1 for Trial 2. Both CIM and AG also significantly reduced NO3-concentrations in plasma and TCF at 12 h postinitiation (Trials 1 and 2). Thus CIM, at the doses studied, was capable of reducing NO3- concentrations in this model as effectively as AG, a relatively specific inhibitor of iNOS activity.

Bioavailability of intranasal scopolamine in normal subjects

The bioavailability of scopolamine in three dosage forms was compared in 12 healthy nonsmoking male volunteers. Subjects received 0.4-mg doses of scopolamine bromide in intravenous (i.v.), intranasal (i.n.), or oral (p.o.) dosage forms on three occasions, with at least 2 weeks separating the doses. Scopolamine concentrations in plasma were determined with a combined reverse-phase liquid chromatographic-radioreceptor binding assay. Saliva volume and flow rate and percent suppression of control flow rate were determined from each sample. Absorption after i.n. and po scopolamine administration was rapid; plasma concentrations [1680 (i.n.) and 164 pg/mL (p.o.)] peaked within 1 h of dosing [0.37 (i.n.) and 0.78 h (p.o.)], respectively. i.n. and i.v. scopolamine suppressed salivary flow rate to similar extents (95% and 99.7%), respectively. Times to reach maximum effect were 1.05 and 0.27 h after i.n. and i.v. dosage, respectively. Absolute intranasal bioavailability, calculated from the area under the drug concentration vs time curve, was found to be significantly greater than that of p.o. scopolamine (83% vs 3.7%, p < 0.05). The i.n. route may provide a noninvasive, reliable, fast, and effective route for administering scopolamine.

Pharmacokinetics and bioequivalence of parenterally administered doramectin in cattle

Plasma concentrations of doramectin in 40 cattle dosed by subcutaneous (sc) or intramuscular (i.m.) injection (200 micrograms/kg) were compared to assess the bioequivalence of the two routes of administration. Peak concentration (Cmax), and areas under the concentration curve (AUC0-infinity) were determined from plasma concentrations. Animals treated by the sc route showed a mean AUC0-infinity of 457 +/- 66 ng.day/mL (+/- SD) and a mean Cmax of 27.8 +/- 7.9 ng/mL. Results from the i.m. treatment group showed a mean AUC0-infinity of 475 +/- 82 ng.day/mL and a mean Cmax of 33.1 +/- 9.0 ng/mL. Absorption constants (ka) determined by modelling were 0.542 +/- 0.336 day-1 after sc administration and 0.710 +/- 0.357 day-1 after i.m. administration. The 90% confidence limits on the difference between mean AUC0-infinity values for the sc and i.m. groups fell within 20% of the mean value for the subcutaneous group. Cmax was somewhat greater for the i.m. route. The 90% confidence limits on the difference in mean In(Tmax +1) also fell within 20% of the mean sc value. Based on this analysis, bioequivalence of the sc and i.m. formulation has been established.

Pharmacokinetics, oral bioavailability and tissue distribution of azithromycin in cats

Azithromycin is the first of a class of antibiotics classified as azalides. In an initial experiment four cats were given a single dose of azithromycin 5 mg/kg orally (p.o.), followed 2 weeks later by a single intravenous bolus (i.v.) dose of 5 mg/kg. Subsequently, six cats were given [14C]azithromycin p.o. in a single dose of 5.4 mg/kg for the study of tissue distribution and metabolism. In both experiments, serial blood samples were collected and the plasma assayed for unchanged azithromycin to determine various pharmacokinetic parameters. After p.o. administration, bioavailability was 58% and absorption rapid with a tmax of 0.85 +/- 0.72 h and a Cmax of 0.97 +/- 0.65 microgram/mL. The harmonic mean terminal t1/2 after i.v. administration was 35 h. Tissue half-lives varied from 13 h in fat to 72 h in cardiac muscle. Three metabolites were identified in bile. Unchanged azithromycin accounted for 100% of the total radioactivity in lung and skin tissues when assayed. In comparison with other species, the bioavailability in cats is higher than in humans but lower than in dogs. As in the dog, > 50% of the azithromycin-related material in feline bile was unchanged azithromycin.

The effects of experimentally induced bronchopneumonia on the pharmacokinetics and tissue depletion of gentamicin in healthy and pneumonic calves

The effects of a bovine bronchopneumonia model on the pharmacokinetics and tissue residue depletion profiles of gentamicin in calves weighing 90-140 kg was explored. Two groups of heifer calves were used. The first was a normal group, while the second group had bronchopneumonia induced. A scoring system was developed to evaluate the extent of disease in the groups. A bimodal distribution of the serum pharmacokinetic parameters in the pneumonic group was caused by the effects of dehydration. When the severely dehydrated calves were omitted from the analysis, serum clearance of gentamicin was significantly higher in the pneumonic group than in the normal group (P less than 0.05). The pharmacokinetic equations used to fit the tissue concentrations varied from tissue to tissue and between groups. Because the best equation of a particular tissue's concentrations varied between groups, withdrawal periods, which are normally determined in healthy animals, may be inappropriate in diseased animals. Addition of several parameters (serum creatinine, serum urea nitrogen, albumin, fibrinogen, and total protein concentrations, white blood cell counts, central fluid volume, volume of distribution at steady state, area under the serum concentration vs time curve, serum clearance, and elimination rate constant) to these tissue-depletion models using multiple regression improved the prediction of a concentration in a given tissue.

Tissue concentrations of clindamycin after multiple oral doses in normal cats

Eighteen normal cats were randomly allocated into two blocks with three treatment groups and dosed orally with clindamycin aqueous solution for 10 days at a dosage rate of 5.5 mg/kg twice daily (Group 1), 11 mg/kg twice daily (Group 2), or 22 mg/kg once daily (Group 3). At the end of dosing, all cats were killed and tissues were taken for clindamycin concentration analysis. Clindamycin was extracted from tissues using solid-phase extraction columns followed by microbiological assay of clindamycin using a cylinder plate assay using M. luteus. Recovery from each tissue was determined by inoculating known concentrations of clindamycin into drug-naive tissues and comparing the observed concentration from the expected concentration. Confirmation that the bioassay detected clindamycin and not N-desmethylclindamycin, its active metabolite, was done using gas-chromatography-mass-spectrometry. Concentrations were highest in the lung, with tissue:serum ratios greater than 3 in all groups. Concentrations were higher in Group 3 than Group 1 (P less than 0.05). Only liver concentrations in Group 3 were statistically higher than in Group 2, although all tissues except bone marrow and CSF had numerically higher concentrations in Group 3 than Group 2. The tissue:serum ratio was greater than 1 in all tissues studied except bone, cerebrospinal fluid, brain, and skeletal muscle.

Extraction methods for quantitation of gentamicin residues from tissues using fluorescence polarization immunoassay

Sodium hydroxide digestion of unhomogenized kidney and skeletal muscle for 20 min at 70 degrees C was a superior method for extracting gentamicin from tissues, compared with simple homogenization, trichloroacetic acid precipitation of homogenized tissue, and sodium hydroxide digestion of homogenized tissue. Fluorescence polarization immunoassay was used to quantitate gentamicin. Sodium hydroxide digestion of unhomogenized tissue allowed for the recovery of 90.0 +/- 5.9% (means +/- SD) from renal cortex and 79.9 +/- 3.5% from skeletal muscle. The limit of sensitivity was 17.4 ng/g kidney tissue, 15.8 ng/g digested muscle, and 39.0 ng/g digested heart. The within-assay coefficient of variation (CV) at 100 ng/g kidney was 9.2%; at 500 ng/g kidney, the CV was 2.5%; and at 2000 ng/g kidney, the CV was 1.5%. The between-assay coefficient of variation was less than 7.5% for all concentrations from kidney, and the 99% confidence interval at 100 ng/g kidney was 71.7-112.4 ng gentamicin/g kidney. The within-assay coefficient of variation (CV) at 100 ng/g muscle was 15%; at 500 ng/g muscle, the CV was 2.6%; and at 2000 ng/g muscle, the CV was 2.3%. The between-assay coefficient of variation was less than 15% for all concentrations from muscle, and the 99% confidence interval at 100 ng/g muscle was 72.5-136.8 ng gentamicin/g muscle. Gentamicin-free milk could be distinguished from milk containing gentamicin concentrations of 10 ng/mL milk with 95% confidence, and from milk containing concentrations of 30 ng gentamicin/mL milk with 99% confidence. Quantitative results at or below the tolerance level can be obtained within 90 min of sample acquisition using these extraction and assay methods.

Oral clindamycin disposition after single and multiple doses in normal cats

Eighteen normal cats were randomly allocated into three treatment groups and dosed with clindamycin aqueous solution for 10 days at a dosage rate of: (1) 5.5 mg/kg b.i.d.; (2) 11 mg/kg b.i.d.; or (3) 22 mg/kg once daily. Serum disposition of clindamycin was determined after the first and last dose of clindamycin was given, and was analyzed using model-independent pharmacokinetics by both the trapezoidal rule method and the predictive equation method. Complete blood counts and clinical chemistries were determined before and after the study. The trapezoidal rule method produced similar mean results with much less variance than the predictive equation method. Mean residence time was longer (P less than 0.05) after the high dose (393 +/- 77 min) than after either the low or medium doses (276 +/- 51 and 274 +/- 45 min, respectively). Oral volume of distribution (Vd(ss)/F) after the high dose (3.06 +/- 0.92 l/kg) was larger (P less than 0.05) than that after the low or medium doses (1.62 +/- 0.30 and 1.76 +/- 0.53 l/kg, respectively). Oral Vd(ss)/F was significantly smaller (P less than 0.001) after the last dose than after the first dose when analyzed by treatment group. Significant (P less than 0.01) decreases in the leukogram and erythrogram were observed, due to the large amount of blood collected for drug analysis. No clinical signs of drug intoxication were observed, and no drug-related necropsy findings were found.